“The W54 warhead used on the Davy Crockett weighed just 51 pounds and was the smallest and lightest fission bomb (implosion type) ever deployed by the United States, with a variable explosive yield of 0.01 kilotons (equivalent to 10 tons of TNT, or two to four times as powerful as the ammonium nitrate bomb which destroyed the Alfred P. Murrah federal building in Oklahoma City on April 19, 1995), or 0.02 kilotons-1 kiloton. A 58.6 pound variant?the B54?was used in the Special Atomic Demolition Munition (SADM), a nuclear land mine deployed in Europe, South Korea, Guam, and the United States from 1964-1989.”
Small nukes do exist, and the marketing folks can package them in many ways.
Fifty one pounds is without shielding. I actually saw one when I was in the Air Force.
All that you have mentioned were prototypes, one-offs, or experiments, except for the land mine, which was deployed in small numbers and never assembled into a workable configuration outside of the US. It was considered too dangerous to use offensively, and was slated as a denial weapon. Also, all of these "projects" were initiated in the 1950's during a period of great advancement in nuclear weapons design in the US.
None of these lightweight and advanced implosion designs could be built by terrorists with a 3rd grade education using a hammer and a grenade pin in some cave somewhere, and none of these designs exist in any arsenal today. Security, storage, and usability make them impractical footnotes of history, nothing else.